1. Field of the Invention
The present invention generally relates to an intermediate-frequency coupled circuit for a television tuner (hereinafter referred to as an IF coupled-circuit), and particularly relates to an IF coupled-circuit in which there is no inductor between itself and a video-signal-selection filter connected thereto, and which has an impedance-matching function and a trapping function for removing an adjacent-channel frequency component.
2. Description of the Related Art
Hitherto, known television tuners which are used in the PAL (Phase Alternating by Line) system adopted in Europe etc. as the color television standard comprise an intermediate-frequency amplifying stage (hereinafter referred to as the IF amplifying stage), a video-signal-selection filter, and an IF coupled-circuit. The IF coupled-circuit is disposed between the IF amplifying stage and the video-signal-selection filter, for removing the frequency component of a channel lower than and adjacent to a selected receiving channel. The IF coupled-circuit may comprise an M-derived trap circuit having a parallel circuit wherein an inductor and a capacitor are connected in parallel, or may comprise a bridged-T trap circuit, for removing the frequency component of the channel lower than and adjacent to the selected channel.
FIG. 3 shows a first exemplary configuration of a television tuner comprising the above-described IF coupled-circuit. This IF coupled-circuit comprises an M-derived trap circuit therein.
This television tuner shown in FIG. 3 comprises an IF coupled-circuit (IF coupled part) 31, a radio-frequency amplifying stage (RFAMP) 32 (hereinafter referred to as the RF amplifying stage 32), a radio-frequency mixing stage (MIX) 33 (hereinafter referred to as the RF mixing stage 33), an intermediate-frequency tuning stage (IF tuner) 34 (hereinafter referred to as the IF tuning stage 34), an intermediate-frequency amplifying stage (IFAMP) 35 (hereinafter referred to as the IF amplifying stage 35), a surface-acoustic-wave filter (SAW) 36 (hereinafter referred to as the SAW filter 36) functioning as a video-signal-selection filter, a receiving antenna 37 (hereinafter referred to as the antenna 37), a video-signal output terminal 38, and an intermediate-frequency-signal output terminal cum intermediate-frequency testing terminal 39 (thereinafter referred to as the IF testing terminal 39).
The IF coupled-circuit 31 comprises a first inductor 311, a first capacitor 312, a second capacitor 313, and a second inductor 314. The first inductor 311 and the first capacitor 312 form a trap circuit for removing the frequency component of a channel lower than and adjacent to the selected channel. The second inductor 314 is provided for matching the impedances of the IF testing terminal 39 and the SAW filter 36.
The input terminal of the RF amplifying stage 32 is connected to the antenna 37, and the output terminal thereof is connected to the input terminal of the RF mixing stage 33. The output terminal of the RF mixing stage 33 is connected to the input terminal of the IF tuning stage 34. The output terminal of the IF tuning stage 34 is connected to the input terminal of the IF amplifying stage 35. The output terminal of the IF amplifying stage 35 is connected to the input terminal of the IF coupled-circuit 31. The output terminal of the IF coupled-circuit 31 is connected to the input terminal of the SAW filter 36, and the output terminal of the SAW filter 36 is connected to the video-signal output terminal 38.
One end of each of the first inductor 311 and the first capacitor 312 is connected to the input terminal of the IF coupled-circuit 31, and the other ends thereof are connected to one end of the second capacitor 313 and to the IF testing terminal 39. One end of the second inductor 314 is connected to the one end of the second capacitor 313 and to the IF testing terminal 39, and the other end thereof is connected to the output terminal of the IF coupled-circuit 31. The other end of the second capacitor 313 is connected to ground.
The operation of the above-described television tuner will now be briefly described.
When the antenna 37 receives TV waves, the antenna 37 transmits the received waves to the RF amplifying stage 32 as a radio-frequency signal (hereinafter referred to as the RF signal). The RF amplifying stage 32 amplifies the RF signal to a predetermined level, and transmits the RF signal to the RF mixing stage 33. The RF mixing stage 33 mixes the RF signal with a local-oscillation signal to form a mixed-frequency signal, and transmits the mixed-frequency signal to the IF amplifying stage 35. The IF amplifying stage 35 selects an IF signal from the transmitted mixed-frequency signal, amplifies the IF signal, and transmits the amplified IF signal to the IF coupled-circuit 31. Upon receiving the IF signal, the trap circuit in the IF coupled-circuit 31 traps and removes the frequency component of the channel lower than and adjacent to the selected channel. Subsequently, the IF signal is transmitted to the IF testing terminal 39, and is transmitted to the SAW filter 36 via the second inductor 314. The SAW filter 36 extracts a video signal from the transmitted IF signal, and transmits the video signal to the video-signal output terminal 38.
FIG. 4 shows a second exemplary configuration of a television tuner comprising the above-described IF coupled-circuit. This IF coupled-circuit comprises a bridged-T trap circuit therein. This drawing shows only the IF coupled-circuit and the other nearby circuits. The circuits that have already been illustrated in FIG. 3 are designated by like reference numerals.
As shown in FIG. 4, this television tuner comprises an IF coupled-circuit (IF coupled part) 41, an intermediate-frequency amplifying stage (IFAMP) 35 (hereinafter referred to as the IF amplifying stage 35), a surface-acoustic-wave filter (SAW) 36 (hereinafter referred to as the SAW filter 36) functioning as a video-signal-selection filter, a video-signal output terminal 38, and an intermediate-frequency-signal output terminal cum intermediate-frequency testing terminal 39 (hereinafter referred to as the IF testing terminal 39). The other elements are not shown in this figure.
The IF coupled-circuit 41 comprises a first capacitor 411, a second capacitor 412, a third capacitor 413, a resistor 414, a first inductor 415, and a second inductor 416. The second capacitor 412, the third capacitor 413, the resistor 414, and the first inductor 415 form a trap circuit for removing the frequency component of the channel lower than and adjacent to the selected channel. The second inductor 416 is provided for matching the impedances of the IF testing terminal 39 and the SAW filter 36.
One end of each of the first capacitor 411, the second capacitor 412, and the resistor 414 are connected to the input terminal of the IF coupled-circuit 41 and to the IF testing terminal 39. The other end of the first capacitor 411 is connected to ground. The other end of the second capacitor 412 is connected to one end of the third capacitor 413 and to one end of the first inductor 415. The other end of the first inductor 415 is connected to ground. The other ends of the resistor 414 and the third capacitor 413 are connected to one end of the second inductor 416. The other end of the second inductor 416 is connected to the output terminal of the IF coupled-circuit 41.
The operation of the above-described television tuner will now be briefly described. However, the operation of this television tuner is the same as in the case of the television tuner shown in FIG. 3 except for the operation performed in the IF coupled-circuit 41. Therefore, only the operation performed by the IF coupled-circuit 41 is described below.
When the IF coupled-circuit 41 receives an IF signal, the trap circuit traps and removes the frequency component of the channel lower than and adjacent to the selected channel from the IF signal. Subsequently, the IF signal is transmitted to the IF testing terminal 39, and is transmitted to the SAW filter 36 via the second inductor 416. The SAW filter 36 extracts a video signal from the transmitted IF signal, and transmits the video signal to the video-signal output terminal 38.
The known IF coupled-circuit 31 and the known IF coupled-circuit 41 have many drawbacks, which are described below.
The IF coupled-circuits 31 and 41 must comprise the second inductor 314 and the second inductor 416, which are connected in series to the input terminal of the SAW filter 36, for matching the impedances of the output terminal of the IF amplifying stage 35 and of the input terminal of the SAW filter 36. Accordingly, the number of parts is increased.
The output terminal of the IF coupled-circuit 31 and the IF testing terminal 39 are connected to each other with a low impedance. Subsequently, there is an increased possibility that an integrated circuit (not shown) having the IF amplifying stage 35 therein will be destroyed by an electrostatic discharge (ESD) due to the low impedance.
In the IF coupled-circuit 31, the frequency component of the channel lower than and adjacent to the selected channel is trapped by the trap circuit. When the attenuation of such a frequency component is increased, the attenuation of a video carrier frequency component of the selected channel is also increased.
In the IF coupled-circuit 41, the frequency component of the channel lower than and adjacent to the selected channel is also trapped by the trap circuit. When the resistance of the resistor 414 is increased to increase the attenuation of the trapped frequency component, the attenuation of the frequency component of the selected channel is also increased due to the increased resistance.